The invention relates to a method for executing a grade change in a paper machine, in which method target ramps are determined in advance for controlled variables of the process, which are ramped according to said target ramps during the grade change.
Further, the invention relates to an apparatus for executing a grade change in a paper machine, the apparatus comprising control means that contain target ramps, which have been defined in advance for the controlled variables of the process and according to which the variables are ramped during the grade change.
A grade change in a paper machine means changing the paper grade currently produced into another grade of paper. A grade change is carried out by simultaneously changing different process variables, such as basis weight and moisture, to correspond to the target values of the new paper grade. The change is executed while the paper web is running through the machine. The product produced during a grade change usually ends up as broke and therefore grade changes should be as fast as possible. Due to the complicated nature of the process and the interdependence of the different variables, a grade change is very difficult to execute. The runs of different paper grades to be produced are often rather small, which results in frequent grade changes, and on the other hand the running speeds of paper machines are high and therefore the time used for a grade change should be minimized. Grade changes should not produce breaks in the paper web either.
U.S. Pat. No. 3,886,036 discloses an open loop grade change solution, wherein target ramps are determined in advance for the controlled variables of the process, such as machine speed, stock flow, headbox pressure and steam pressure, and the grade change is executed according to these target ramps. Determining the target ramps requires the development of process models. Further, the open loop arrangement is criticized in the US patent since in practice the process models depend on assumptions made during the modelling, which means that when the conditions change slightly the assumptions are no longer valid and the grade change is not very successful. Another problem set forth in the patent is for example that a slight change in the properties of the pulp causes such an alteration in the conditions that the grade change model does not work well anymore. As a solution to these problems, the US patent discloses a closed loop grade change arrangement, which suggests combining the control loops for basis weight and moisture such that adjusting one variable does not cause a great change in the other variable. According to the U.S. patent, such a grade change can only be carried out when the following restrictions apply:
1) the steam pressure is kept constant during the grade change
2) the machine speed is calculated by the control loops, i.e. in practice the only variable altered during a grade change is the basis weight. Such a grade change is called dryer limited grade change. A change in the basis weight is also connected to cause a corresponding change in the slice of the headbox. The machine speed is adjusted to maintain the moisture at a desired value. Such closed loop grade changes do not operate smoothly, which means that a grade change takes too much time.
Japanese patent publication 6,071,793 discloses an apparatus for controlling the changing of a paper grade in a paper machine, in which apparatus the speeds of different parts of the paper machine are controlled by altering the draw with respect to a change in the basis weight. The optimum model is calculated and it is optimized during the grade change. It is not disclosed how the other process variables are taken into account. The apparatus according to the Japanese patent publication might make it possible to optimise the draw, but this arrangement is not good enough considering the speed and overall control of the grade change.
The purpose of the present invention is to provide a method and an apparatus which provide a fast and controlled grade change in a paper machine.
The method according to the invention is characterized by collecting data about the grade changes already executed and thereafter determining grade change models by selecting the data about successful grade changes and by determining the target ramps by means of the grade change models.
Further, the apparatus according to the invention is characterized in that the target ramps supplied to the control means have been determined by means of grade change models defined on the basis of successful grade changes.
The basic idea of the invention is that a grade change is executed by determining target ramps in advance for the controlled variables of the process by means of grade change models determined for the process output variables, and the control variables are ramped during the grade change in accordance with the determined target ramps. Further, it is essential that the grade change models are determined by collecting data about the grade changes that have already been executed and by thereafter using as grade change models the grade change models determined on the basis of the successful grade changes. Individual grade change models are defined for different types of changes, for example an increase or decrease in the basis weight. Further, the idea of a preferred embodiment is that during a grade change the target moisture is predicted through modelling by taking into account the effective production rate and the effective steam pressure and by comparing the estimate to the moisture measured, whereupon the feedback provides a disturbance variable which is monitored throughout the grade change, which means that external disturbance should be eliminated during the grade change or the ramps are corrected by the disturbance detected.
The invention has the advantage that a grade change can be executed rapidly and the process is well controlled during the grade change so that there are for example very few breaks. By estimating the moisture it is possible to determine the grade change model more accurately and to eliminate, if necessary, changes in the original values. The arrangement according to the invention enables the activation of a grade change before the run of the previous grade of paper has been completed and ensures that the paper moisture does not change too much during the grade change and does not thus prevent the operation of the subsequent process steps. The invention provides a very rapid and accurately controlled grade change that is executed by means of simple grade change models, which means that the modelling and tuning is also relatively easy.